The link between the H1N1 vaccine and Guillain-Barré first emerged in 1976, during the U.S.’s last urgent national campaign to immunize citizens against a fast-acting flu.

At that time, the risk of the disease, which is a rare but serious immune disorder in which the body’s own immune cells start to attack nerves, leading to muscle weakness, paralysis and even death, was concerning enough that the vaccination program was halted. H1N1 immunization was called for again in 2009, however, when threat of a related pandemic strain of influenza against which most of the population wasn’t immunized, called for a massive vaccination campaign to avoid a pandemic. Now the latest study on adverse events reported from those vaccinations, published in the Lancet, shows that the risk of Guillain-Barré remained.

The data included adverse events reported to six different systems, including the core vaccine safety datalink, new surveillance systems established by Medicare and the U.S. Department of Defense and Veterans Affairs, as well as a monitoring system specifically created to track adverse events related to the 2009 H1N1 immunization campaign. Researchers from the National Vaccine Program Office of the U.S. Department of Health and Human Services analyzed the data from 23 million people who were vaccinated as part of that effort and found 61 million cases of H1N1, 274,000 hospitalizations and 12,470 deaths related to the pandemic flu. Of those who were immunized, 77 developed Guillain-Barré, some as long as 91 days after getting their shot. Since the disorder normally affects about one person in 100,000, this results in an additional 1.6 cases of Guillain-Barré syndrome per one million people vaccinated — still a small risk.

This rate is similar to that found in other studies that have studied the link between the vaccine and the autoimmune disorder. Last summer, a group of scientists in Quebec analyzed the risk of Guillain-Barré syndrome in millions of people given the vaccine in Canada between 2009 and 2010 and reported 83 confirmed cases of Guillain-Barré, including 25 cases among people who received the H1N1 vaccine.

The connection between H1N1 vaccines and Guillain-Barré first emerged during the swine flu pandemic in 1976, when public health experts ordered mass immunizations against an H1N1 influenza virus related to the one seen in 2009; health officials saw slight increases in the syndrome following the mass inoculations.

After reviewing data on the link, the Institute of Medicine concluded that the vaccine was responsible for the increased risk of Guillain-Barré, and that some people were more susceptible to flu-shot-triggered cases than others. Scientists said that by producing antibodies against the flu virus in response to the vaccine, some people may inadvertently generate antibodies against their own cells as well, causing the disorder.

Older adults are more likely to develop the disorder and about 80% recover completely with the proper treatment. According to the Centers for Disease Control, an estimated 3,000 to 6,000 Americans develop the disorder each year, regardless of whether they received a flu vaccine.

While there is a small risk of paralysis related to Guillain-Barré, the incidence of the disease arising from the H1N1 vaccine is so small that public health experts say the benefits of immunization for most people against pandemic flu far outweighs this small risk. (The exception are people with a history of Guillain-Barré.) In 2009, for example, officials estimate that the H1N1 vaccine prevented anywhere from 700,000 to 1.5 million cases of influenza, kept 4,000 to 10,000 people from needing hospitalization for flu-related symptoms, and prevented 200 to 5oo deaths. So while doctors, patients and public health officials should be aware of all the risks of the vaccine, these risks are not sufficient to deter vaccination.